[DGD] Nested compilation, when allowed

[Raymond Jennings]

Just curious, but why/what is the purpose of forbidding nested compilations?

At first, it was only allowed within driver::inherit_program, but I
think it was loosened to allow the driver object to do any compilation
it wanted at or under driver::inherit_program

What is the purpose of forbidding any object but the driver object
from compiling in this case?  I imagine that the driver object could
in theory call another object to do it, were it not for this policy.

Commit on 2014-Aug-05 is where it got loosened for context (no pun intended).